A number of different so-called diagonal cutters are known which are disposed in the dryer section of a paper machine, in particular at the downstream end or dry end of the dryer section. In these prior art diagonal cutters a pin fixed to a mechanical traverse has been used as the tool for cutting the leader of the web and for diagonal shearing or cutting of the web. As the cutting pin is contacting the surface of the drying cylinder it is disadvantageous in this arrangement that the pin scratches the cylinder surface and soon becomes dull seriously affecting the quality of the cut.
It is also known to use, as a diagonal cutter, a device similar to a circular saw mounted on a traversing carriage or to carry out the diagonal or oblique cutting manually with a suitable knife.
A device for cutting the leader of a paper web is known from applicant's U.S. Pat. No. 4,566,944, the entire disclosure of which is hereby incorporated herein by reference. This device comprises a beam mounted in connection with a multi-cylinder dryer of a paper machine in a direction transverse to the path of the paper. The beam is provided with a traversing carriage on which a web-cutting blade is mounted. The apparatus of U.S. Pat. No. 4,566,944 includes a carriage bearing the cutting blade while the substantially plain carriage face is provided with means for blowing air against the web to stabilize the web and prevent fluttering thereof. The blow means include oppositely disposed nozzle slots or suitable nozzle holes within the carriage.
A problem has, however, arisen in the prior art methods and devices when utilized in connection with a single-wire draw in a multi-cylinder dryer of a paper machine or a finishing machine. A multi-cylinder dryer with a single wire draw generally comprises two rows of heated drying cylinders, one row disposed above the other, or one row of heated drying cylinders and one row of unheated leading rolls or cylinders with and without vacuum. Between the rows of cylinders and/or leading rolls, in each group of single-wire draw, the web is supported by the drying wire or drying felt so that the cylinders in one row of cylinders, as a rule in the row of upper cylinders, are placed outside the loop of the drying wire and the lower cylinders or lead rolls are located inside the loop of the drying wire or felt. Thus, in a single-wire draw, the web of paper is also supported on the usually straight runs between the rows of respective cylinders. In the case of twin-wire draws, the paper web has a free-unsupported run between the two respective cylinders or groups of cylinders so that the prior-art diagonal cutters can be used at this location, as is shown in FIG. 1 of applicants' above-mentioned U.S. Pat. No. 4,566,944.
Since, in the case of a single-wire draw, the paper web is supported by one surface of the drying wire, the prior art diagonal cutters cannot be utilized on the runs between the rows of cylinders in these single wire dryers. This has presented a substantial obstacle and limitation for the installation of diagonal cutters in the very position which is the most suitable in view of the operation and functioning thereof.
An object of the present invention is thus to provide a method and apparatus for cutting a paper web wherein unsupported draws are minimized thereby avoiding breaks of the paper web due to fluttering and like causes.
It is a further object of the present invention to provide a method and apparatus for the cutting of the lead-in strip or leader of a paper web so as to permit the threading of the paper web without the use of a guiding rope.
To achieve these and other objectives as will become apparent further below, the present invention includes a method of cutting a paper web in a group of drying cylinders provided with a single-wire draw, the method comprising separating the drying wire from the paper web by one or more guide rolls or equivalent means thereby creating an open draw for the web, carrying out the cutting of the web including the diagonal cutting of the leader of the web within the open draw of the web, and, after said open draw, converging and rejoining the drying wire and the web over the following leading or drying cylinder.
The means equivalent to the above-mentioned guide rolls include, for example, known static leading or guiding devices based on contact-free airborne support.
The apparatus in accordance with the present invention includes a transversely disposed beam with the cutting element, and above said beam, one or two guide rolls for the drying wire. In case of two rolls, the first roll is placed in a position in which the drying wire and the web, which jointly arrive from the upper cylinder, are separated from each other and the drying wire is guided further around the first guide roll, and transferred to the second guide roll and, from there transferred to the lower cylinder to meet the web, after the open draw, on the surface of the lower cylinder. The transverse beam and its cutting blade is disposed in the vicinity of the free run or open draw of the web.
The construction of the cutter utilized in the present invention may be, for example, similar to that described in applicants' above-mentioned U.S. patent and which includes a rotatable or fixed blade, or some other, corresponding prior art diagonal cutter, such as a device for cutting the web with a water jet.
In general, the method of the present invention is utilized at the downstream or dry end of a group of dryers provided with a single-wire draw in a paper machine or board machine, and preferably, before the upper drying cylinder from whose lower circumference the rest of the paper web, from which the leader has been cut, is removed by a doctor and guided into the pulper located underneath the dryer. The invention may also be applied to various multi-cylinder dryers provided with a cylinder group with single-wire draw in paper or board finishing machines.
The method and the apparatus in accordance with the present invention also permits the rope-free threading of the web and can be used for cutting the leader of the web in geometries of multi-cylinder dryers such as applicants' "SYM-RUN" (trademark) dryer or the like.